This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Bacteria secrete a variety of signaling molecules that allow them to coordinate gene expression and behave as multicellular organisms. In response to these 'quorum sensing'or 'autoinducer'signals, such medically important phenotypes as virulence factor expression, biofilm formation, and drug resistance are modulated in a population wide manner. The enzyme 5'Methylthioadenosine / S-adenosylhomocysteine nucleosidase (MTA/SAH nucleosidase, MTN) occupies a central place in the biosynthetic pathways that lead to both autoinducer I (AI-1) and autoinducer II (AI-2) formation. In addition, MTN governs a crucial step in the recycling of methionine and adenine consumed during S-adenosylmethionine dependent polyamine synthesis and methylation reactions. Pharmacologic or genetic inhibition of MTN should block methionine and purine salvage, cause growth delays through the accumulation of inhibitory MTA and SAH nucleosides, and interfere with autoinducer synthesis and downstream signal dependent processes. To examine the role of this enzyme in nutrient salvage and signaling pathways, MTN knock-out strains of E. coli (O157:H7) and Klebsiella pneumoniae will be created and studied for alterations in growth (rate, carbon utilization, biofilm formation), attenuation of mammalian cell invasion in in vitro models of infection, and in murine models of in vivo colonization and virulence. Proteomic and metabolomic adaptations to MTN gene deletion will also be examined by LC/MS and NMR to further characterize the molecular consequences of enzyme interruption and explain the basis for observed alteration in phenotype. Ultimately, these experiments should underscore the importance of cellular signaling in bacteria as a target for novel antibiotic development.